Polymeric materials have been classically considered to be passive. A polymeric material which can exhibit an active response to its environment, including temperature, has a wide range of potential applications based on the response, type, factor and the physical properties of the polymer-based system.
Temperature-dependent conductivity and associated “switching” responses in solid-state bulk polymeric composite materials are known in the literature and find application in many fields including sensing and actuation. There abound various examples of so-called positive temperature coefficient (PTC) and negative temperature coefficient (NTC) materials, which exhibit often abrupt increases or decreases in electrical conductivity as a function of temperature. The switching response of known PTC and NTC material are exclusively based on a physical perturbation of a conductive phase within a non-conductive matrix and the associated reversible or irreversible breakdown or establishment of a percolative network of conductive nodes, as a consequence of thermal expansion, contraction or phase change in the non-conductive phase.
One example of such a system with an irreversible or high hysteresis PTC response would be the thermal fuse type system whereby a bound, percolative 3D network of conductive particles (e.g., carbon black) in a non-conductive polymer matrix suffers an irreversible breakdown in percolation with the consequential loss of conductivity of the device, when the system is exposed to temperatures in excess of a predefined expansion limit of the composite. See Feng et. al., Polymer, 41(12):4559-4565 (2000).
Less common are reversible NTC response materials. Xiang et. al., Macromol. Mater. Eng., 294:91-95 (2009) recently demonstrated a polymeric foam/CNT composite device that exhibited temperature dependent increases in electrical conductivity as a result of expansion of the gas-filled closed cell structure of the polymeric foam increasing the contact order of a discrete conductive CNT phase within the polymer matrix. While such demonstrations are both useful and relevant, the primary mode of switching response in such materials was the physical onset or breakdown of a percolative particle dispersion in a contiguous non-conductive matrix. As such the magnitude, repeatability and timescale of the temperature-dependent response are questionable, due in the mainstay to the complexities and uncertainties associated with the physical and often fractal filler networks and their associated aging response (e.g., Mullin effect and Payne effect). However, rapid response, large magnitude reversible and hysteresis free NTC type behavior is beyond the scope of such known “percolative” switching composite devices.